• Publications of The Korean Astronomical Society
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• v.32 no.1
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• pp.117-121
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• 2017

We show Akari data, Herschel data and data from the SCUBA2 camera on JCMT, of molecular clouds. We focus on pre-stellar cores within the clouds. We present Akari data of the L1147-1157 ring in Cepheus and show how the data indicate that the cores are being externally heated. We present SCUBA2 and Herschel data of the Ophiuchus region and show how the environment is also affecting core evolution in this region. We discuss the effects of the magnetic field in the Lupus I region, and how this lends support to a model for the formation and evolution of cores in filamentary molecular clouds.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.73.1-73.1
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• 2016

We present preliminary results of the submillimeter continuum observations of 14 Planck Galactic Cold Clumps (PGCCs), located in the ${\lambda}$ Orionis Complex. This region is the nearest large HII region, which is an ideal site for a study of the stellar feedback to its surroundings. We observed 14 PGCCs with JCMT/SCUBA-2 and used J=1-0 transitions of CO isotopologues from the PMO mapping observation. Several sub-clumps toward three PGCCs were detected at $850{\mu}m$. In order to examine whether these clumps can be candidates for pre-stellar cores, we compared each clump mass calculated from the $850{\mu}m$ continuum map to its Virial mass and Jeans mass calculated from the $^{12}CO$ and $C^{18}O$ (1-0) spectra, respectively. All clumps have masses smaller than their Virial and Jeans masses, indicating that none of them are gravitational bound and thus in the pre-stellar core stage. Also, the CO depletion factor, which has been derived from the dust continuum and the $C^{18}O$(1-0) line and can be an indicator of core evolution, toward the clumps is in the range of 1 to 5, suggesting that they are not very evolved dense pre-stellar cores. In addition, within individual PGCCs, we found clear gradients of velocity (${\sim}1km\;s^{-1}\;pc^{-1}$) and temperature (${\sim}10K\;pc^{-1}$) in the $^{13}CO$ (1-0) first moment map and the $^{12}CO$(1-0) excitation temperature map, respectively. This can be attributed to the compression and external heating by the HII region, which may prevent clumps from forming gravitationally bound structures and eventually disperse clumps. These results could be a hint about the negative effect of stellar feedback on core formation.

• Journal of The Korean Astronomical Society
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• v.37 no.4
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• pp.265-268
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• 2004

Molecular clouds present many levels of structure, including clumps and cores of varying size and density. We present a brief summary of these cores, describing their observed physical properties and their place in the star formation process. We conclude with some speculation about pre-proto-stellar stages of molecular cores and the observational challenges in their observation.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.80.1-80.1
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• 2017

We present preliminary results of KVN single dish telescope observations of 80 dense cores in the Orion molecular cloud complex which contains the Orion A, B, and ${\lambda}$ Orionis cloud. We investigate the behavior of the different molecular tracers and look for chemical variations of cores in the three clouds in order to systematically investigate the effects of stellar feedback. The most commonly detected molecular lines (with the detection rates higher than 50%) are N2H+, HCO+, H13CO+, C2H, HCN, and H2CO. The detection rates of dense gas tracers, N2H+, HCO+, H13CO+, and C2H show the lowest values in the ${\lambda}$ Orionis cloud. We find difference between molecular D/H ratios and N2H+/H13CO+ abundance ratios towards different clouds, and between protostellar cores and starless cores. Eight starless cores in the Orion A and B clouds exhibit high deuterium fractionations, larger than 0.10, while in the ${\lambda}$ Orionis cloud, no cores reveal the high ratio. These chemical properties could support that cores in the ${\lambda}$ Orionis cloud are affected by the photo-dissociation and external heating from the nearby H II region, which is a hint of negative stellar feedback on core formation. The striking difference between the [N2H+]/[H13CO+] ratios leads us to suggest that there are significant evolutionary differences between the Orion A/B and ${\lambda}$ Orionis clouds. In order to examine whether starless cores can be candidates of pre-stellar cores, we compared the core masses estimated from the 850 um emission to their Virial masses calculated from the N2H+ line data and find that most of them are not gravitationally bound in the three clouds.

• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.157-160
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• 2007

I present a model to explain the mass segregation and shallow mass functions observed in the central parts of starburst stellar clusters. The model assumes that the initial pre-stellar cores mass function resulting from the turbulent fragmentation of the proto-cluster cloud is significantly altered by the cores coalescence before they collapse to form stars. With appropriate, yet realistic parameters, this model based on the competition between cores coalescence and collapse reproduces the mass spectra of the well studied Arches cluster. Namely, the slopes at the intermediate and high mass ends, as well as the peculiar bump observed at $6M_{\bigodot}$. This coalescence-collapse process occurs on a short timescale of the order of the free fall time of the proto-cluster cloud (i.e., a few $10^4$ years), suggesting that mass segregation in Arches and similar clusters is primordial. The best fitting model implies the total mass of the Arches cluster is $1.45{\times}10^5M_{\bigodot}$, which is slightly higher than the often quoted, but completeness affected, observational value of a few $10^4M_{\bigodot}$. The model implies a star formation efficiency of ${\sim}30$ percent which implies that the Arches cluster is likely to a gravitationally bound system.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.93-97
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• 2015

How high-mass stars form is currently unclear. Calculations suggest that the radiation pressure of a forming star can halt spherical infall, preventing further growth when it reaches $10M_{\odot}$. Two major theoretical models on the further growth of stellar mass have been proposed. One model suggests the merging of less massive stellar objects, and the other is through accretion, but with the help of a disk. Inflow motions are key evidence for how forming stars gain further mass to build up massive stars. Recent developments in technology have boosted the search for inflow motion. A number of high-mass collapse candidates were obtained with single dish observations, and mostly showed blue profiles. Infalling signatures seem to be more common in regions which have developed radiation pressure than in younger cores, which is the opposite of the theoretical prediction and is also very different from observations of low mass star formation. Interferometer studies so far confirm this tendency with more obvious blue profiles or inverse P Cygni profiles. Results seem to favor the accretion model. However, the evolution of the infall motion in massive star forming cores needs to be further explored. Direct evidence for monolithic or competitive collapse processes is still lacking. ALMA will enable us to probe more detail of the gravitional processes.

• Publications of The Korean Astronomical Society
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• v.29 no.2
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• pp.29-34
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• 2014

We have observed the deuterated methanol, $CH_3OD$, toward the hot core MM1 in the massive star-forming region DR21 (OH) using the Submillimeter Array with a high angular resolution of about 1 arcsecond. The position of the hot core associated with the sub-core MM1a was confirmed to coincide with the continuum peak where an embedded young stellar object is located. The column density of $CH_3OD$ was found to be about $(2{\pm}1){\times}10^{16}cm^{-2}$ toward the MM1a center. The abundance ratio $CH_3OD/CH_3OH$ was measured to be ~ 0.45, which is about the median value for low mass star-forming cores but much larger than those of the massive star-forming cores. The ratio is believed to change depending on, for example, the chemical condition, the temperature and the density of the source. This ratio may further depend on the evolutionary phase especially in the massive-star-forming cores. The sub-core MM1a is thought to be in the very early phase of star formation. This large abundance ratio found in this source indicates that even the massive star-forming cores, during a relatively short period in the very early stage of star formation, may also show a chemical state resulted from the cold and dense pre-collapsing phase, the enhanced deuteration as found in low mass star-forming cores.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.105.1-105.1
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• 2011

We present the modeling results of deuterium fractionation of water ice, $H_2$, and the primary deuterium isotopologues of $H3^+$ in the physical conditions associated with the star and planet formation process. We calculated the deuterium chemistry for a range of gas temperatures (Tgas~10-30 K) and ortho/para ratio (opr ) of $H_2$ based on state-to-state reaction rates and explore the resulting fractionation including the formation of a water ice mantle coating grain surfaces. We find that the deuterium fractionation exhibits the expected temperature dependence of large enrichments at low gas temperature, but only for opr-H2<0.01. More significantly the inclusion of water ice formation leads to large D/H ratios in water ice (${\geq}10^{-2}$ at 10 K) but also alters the overall deuterium chemistry. For T<20 K the implantation of deuterium into ices lowers the overall abundance of HD which reduces the efficiency of deuterium fractionation at high density. Under these conditions HD will not be the primary deuterium reservoir in the cold dense interstellar medium and $H3^+$ will be the main charge carrier in the dense centers of pre-stellar cores and the protoplanetary disk midplane.